1. Field of the Invention:
This invention relates to varifocal optical systems, and more particularly to the variation of the refractive power of at least one lens unit with which the focal length of such system varies.
2. Description of the Prior Art:
In the prior art of varifocal optical systems, when varying the image magnification, it has been the general practice that at least two lens units are made to move along a common optical axis. And determination of the space they occupy is always based on the sum of their total axial movements to avoid mechanical interference therebetween throughout the entire range of variation of the focal length. A limitation was, therefore, laid on the advance in compactness of the lens system. Also because at least one of the lens units has to be moved non-linearly throughout the range, it was difficult to operate the lens units smoothly.
Meanwhile, the inventor of this patent application has proposed an optical system having a lens of variable refractive power in U.S. patent application Ser. No. 556,705 filed Nov. 30, 1983. And the use of this lens makes it possible to eliminate the above-described drawbacks, but in to actually incorporating that lens into the optical system, alternative problems arise.
An object of the present invention is to reduce the number of movable lens units for varying the image magnification with an advantage of deleting the space which would otherwise be necessary for the movable-turned stationary lens unit, whereby a compact optical system as a whole is achieved.
Another object is to eliminate the drawback arising from the use of a lens of variable refractive power as a constitutent element of a thick lens system. In this connection, it should be noted that as the refractive power of the lens varies by varying the radius or radii of curvature of either or both of the surfaces thereof and its axial thickness, the principal points of the lens change their places. And the displacement of the principal points often tends to complicate the structure of the optical system.
Still another object is that the variation of refractive power which contributes mainly to variation of the image magnification is imposed on movement of the lens unit, and the variation of relatively weak refractive power which compensates for the shift of the image plane is imposed on deformation of the lens shape, whereby the image quality is prevented from lowering due to the great degree of distortion of the lens shape.
A further object is to make it possible to perform variation of the image magnification by linearly moving a movable lens unit, whereby a simplification of the lens barrel is achieved.